For dish antennas used to transmit a signal over large distances, the on-axis gain scales as $D^2/ \lambda^2$ or $D^2 f^2 / c^2$ where $\lambda$ and $f$ are the wavelength or frequency and $c$ is the speed of light. All else equal, doubling the frequency means four times the power received by a distant receiver with a fixed receive antenna area.

Question: Excluding optical frequencies, what is the highest frequency that has been used or even tested with spacecraft that could be used beyond Earth orbit? I'm looking for an approximate frequency, not just a letter-code for a band designation.

note 1: reception in Earth orbit (as well as on Earth's surface) counts for this question.

note 2: I've adjusted this question a bit to allow for testing that wasn't necessarily done beyond Earth orbit, as long as the results are applicable and informative toward potential use beyond Earth orbit.

$\begingroup$According to this document spaceacademy.net.au/spacelink/radiospace.htm the upper edge of the radio window of the atmosphere is at 30 GHz. "Above 30 GHz, the lower atmosphere or troposphere, below 10 km, absorbs radio signals due to oxygen and water vapour. Even between 20 and 30 GHz, there are some absorption bands that must be avoided."$\endgroup$
– UweOct 11 '17 at 9:32

$\begingroup$The frequency range of the Ka band is 26.5–40 GHz. The absorption bands (20 to 30 GHz and above 30 GHz) are within the Ka band, not all frequencies of the Ka band may be used for ground to space links.$\endgroup$
– UweOct 11 '17 at 10:35

$\begingroup$I see what you mean. That's why the question asks for "an approximate frequency, not just a letter-code for a band designation."$\endgroup$
– uhohOct 11 '17 at 10:37

1

$\begingroup$According to this paper tele-satellite.com/TELE-satellite-0709/eng/feature.pdf the Ka band is very weather dependent. The ground station should be in a very dry region. The attenuation increases with frequency and rain intensity. Very light rain with less than 5 mm/h is no problem. See figure 4 on the last page. There is much difference between a frequency at the lower or upper border of the Ka band.$\endgroup$
– UweOct 11 '17 at 19:15

At the top end of the lower window, there an experimental effort using the AlphaSat Aldo Paraboni payload to measure space-Earth effects in Ka/Q and lower V bands. The Ka/Q effort is looking at attenuation and propagation around 40GHz. The V effort is looking at 48GHz. There are several paywalled IEEE papers on results, but unfortunately the only non-paywalled sources I can find are just about the equipment tests.

$\begingroup$Inmarsat-4A F4 is in GEO and so it doesn't address the question, but the experiment is still fun to read about. I'd specified "deep space" and "beyond Earth orbit" because I was fairly certain that higher frequencies had beed at least tested locally.$\endgroup$
– uhohApr 2 '18 at 1:48

$\begingroup$Actually I've adjusted the question wording so that this can be the accepted answer. This is really interesting!$\endgroup$
– uhohApr 2 '18 at 6:30

edit: Since I'm also the OP, after accepting @BobJacobsen's answer I've slightly revised the question to allow for another, very informative answer, which addresses testing of a higher frequency band through the Earth's atmosphere, though the testing was done from LEO and not deep space. This answer here is about the highest frequency used or tested from deep space.

I'll post this as a tentative answer. I'm not 100% sure a higher frequency hasn't been tested but it looks like the Deep Space Ka-band frequencies around 32 GHz is the highest that's received substantial, systematic testing and use in deep-space.

Mars Reconnaissance Orbiter (MRO):

Among the highlights of these was setting a single day record for data return from a deep space spacecraft (133 Gbits) achieved during one 10-hour pass; achieving the highest data rate ever from a planetary mission (6 Mbps) and successfully demonstrating Ka-band DDOR.

I'm not sure of the distances, the experiments seem to have been done during the cruise-phase of the mission before orbital insertion.

Juno:

The Ka-band transmissions from the Juno Spacecraft have been systematically studied for an extended period of time. In the IPN Progress Report Cassini Downlink Ka-Band Carrier Signal Analysis. Here's the complete abstract, there is plenty to read in this report:

Lower frequency telemetry bands are becoming more limited in bandwidth with more competition between flight projects and other entities. Higher frequency bands offer significantly more bandwidth and hence the prospect of much higher data rates. Future or prospective flight projects considering Ka-band (32-GHz) telemetry data links are interested in past flight experience with received Ka-band data. Over 10 years of Cassini closed-loop received Ka-band carrier data involving over 2 million individual measurements were acquired at all three Deep Space Network (DSN) sites. We analyzed these data to characterize link performance over a wide range of weather conditions and as a function of elevation angle. Based on this analysis, we have derived a recommendation for telecommunications
link margin for preflight planning purposes. These results suggest that a 4-dB margin will ensure a ~94 percent data return at a minimum 20-deg elevation angle under 90 percent weather conditions at 32 GHz (Ka-band).

$\begingroup$If 40 GHz instead of 32 GHz would be used for down link, the increase of antenna gain (same dish diameter) will be only 1.9 dB. I think to compensate the increased ionospheric and atmospheric attenuation a larger margin will be necessary to get a similar availability of the link.$\endgroup$
– UweOct 13 '17 at 12:19

2

$\begingroup$See this article on the use of W-band. Looks like we won't be seeing W-band used for deep-space anytime soon.$\endgroup$
– Mark AdlerNov 12 '17 at 21:53

$\begingroup$@MarkAdler thanks for the link - good reads! it looks like anything still microwave-like beyond Ka band needs to be received in orbit rather than on the ground, and at that point there doesn't seem to be any reason not to go straight to optical. As far as this question is concerned, if it turns out that there haven't been any spacecraft in deep space with capability to transmit or receive beyond Ka band, that would be an answer I can accept.$\endgroup$
– uhohNov 13 '17 at 2:26